Studies on the biosynthesis of ubiquinone are now focused on the role of polyprenylated derivatives of 3, 4-dihydroxybenzoate as intermediates in the pathway of the conversion of 4-hydroxybenzoic acid to the quinone ring of ubiquinone. We have found that 4-hydroxybenzoic acid can be converted to protocatechuic and vanillic acids. Protocatechuic acid can be prenylated and it can also be converted to ubiquinone in rat heart slices. Vanillic acid however although it can be prenylated does not appear to be converted to ubiquinone in the same system. Thus, it is possible that the pathway then involves only protocatechuic acid or its prenylated product as an intermediate. Our recent studies are focused on the decarboxylation of protocatechuic acid, vanillic acid and 4-hydroxybenzoic acid. Protocatechuic acid appears to be decarboxylated much more rapidly than vanillic acid. The role of enzymes that catalyze reactions analogous to catechol-o-methyl transferase are being examined in relation to these reactions. Further studies have focused on delineating a system which would have some advantages for studying ubiquinone biosynthesis. We have found such a system in the adult rat heart beating cell preparation. We have found that this preparation readily incorporates 4-hydroxybenzoic acid into ubiquinone and is dependent upon cell concentration and time.